Molding is a process by virtue of which a molded article can be formed from molding material by using a molding system. Various molded articles can be formed by using the molding process, such as an injection molding process. One example of the molded article that can be formed, for example, from polyethylene terephthalate (PET) material is a preform that is capable of being subsequently blow-molded into a beverage container, such as, a bottle and the like. Other examples of the molded articles include thin-wall containers (i.e. yogurt containers, cups, etc), medical appliances and the like.
As an illustration, injection molding of PET material involves heating the PET material (ex. PET pellets, PEN powder, PLA, etc.) to a homogeneous molten state and injecting, under pressure, the so-melted PET material into a molding cavity defined, at least in part, by a female cavity piece and a male core piece mounted respectively on a cavity plate and a core plate of a mold. The cavity plate and the core plate are urged together and are held together by clamp force, the clamp force being sufficient to keep the cavity and the core pieces together against the pressure of the injected PET material. The molding cavity has a shape that substantially corresponds to a final cold-state shape of the molded article to be molded. The so-injected PET material is then cooled to a temperature sufficient to enable ejection of the so-formed molded article from the mold. When cooled, the molded article shrinks inside of the molding cavity and, as such, when the cavity and core plates are urged apart, the molded article tends to remain associated with the core piece. Thereafter, the molded article can be ejected off the core piece by use of one or more ejection structures. Ejection structures are known to assist in removing the molded articles from the core halves. Examples of the ejection structures include stripper plates, stripper rings and neck rings, ejector pins, etc.
As generally known in the art, known molding systems can cause a number of visible defects to the molded articles. One such problem is known in the art as “rolled threads” or, in other words, a significant deformation to a neck finish of the preforms.
U.S. Pat. No. 6,558,588 issued to Shioiri et al. on May 6, 2003 discloses a method for controlling an injection molding machine, where a screw is advanced from an injection start position by means of velocity control, and when the screw reaches a control changeover point, pressure control is effected in place of the velocity control to apply pressure to the screw. Preliminary molding is performed to obtain an acceptable product; a total charged amount of resin during the preliminary mold is obtained on the basis of a movement distance over which the screw has moved before reaching the control changeover point during the preliminary mold; and the total charged amount is stored as a changeover target value. During ordinary molding, a total charged amount of resin is obtained on the basis of a movement distance over which the screw advances from the injection start position; and pressure control is started when the total charged amount reaches the changeover target value. Thus, the total amount of resin charged into a mold cavity in each molding cycle becomes constant without being affected by the charging pressure and temperature of the resin.
U.S. patent application bearing a publication number 2005/0053684 published on Mar. 10, 2005 by Pitschebeder et al. discloses an injection molding machine with a mold that can be opened and closed, into the mold cavity of which a fluid molding compound, preferably plastic, can be injected, and with at least one reading device for at least one transponder arranged in the mold cavity.